EQUITY STRUCTURE AND ACCOUNTS - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
AOCI Attributable to Parent [Roll Forward]
Beginning Balance			$ 1,488,146	$ 1,509,554	$ 1,509,554
Other comprehensive income (loss)	$ (2,247)	$ 429	(11,104)	5,181	(2,220)
Exchange of noncontrolling interest for common stock			0	0
Rebalancing of ownership percentage between Company and Operating Partnership			0	0
Ending Balance	1,512,462		1,512,462		1,488,146
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent [Roll Forward]
Beginning Balance			(212)	1,365	1,365
Other comprehensive income (loss)			(9,506)	3,018	(2,915)
Exchange of noncontrolling interest for common stock			(143)	1,422
Rebalancing of ownership percentage between Company and Operating Partnership			6	(102)
Ending Balance	(9,855)	5,703	(9,855)	5,703	(212)
Accumulated Other Comprehensive Income of Noncontrolling Interests
AOCI Attributable to Parent [Roll Forward]
Beginning Balance			116	761	761
Other comprehensive income (loss)			(1,598)	2,163
Exchange of noncontrolling interest for common stock			143	(1,422)
Rebalancing of ownership percentage between Company and Operating Partnership			(6)	102
Ending Balance	(1,345)	1,604	(1,345)	1,604	116
Total Accumulated Other Comprehensive Income
AOCI Attributable to Parent [Roll Forward]
Beginning Balance			(96)	2,126	2,126
Ending Balance	$ (11,200)	$ 7,307	$ (11,200)	$ 7,307	$ (96)